Sputtering targets serve as sources of materials to be sputtered on substrates to form electrodes, gates, wirings, elements, protective films and the like for various semiconductor devices. Sputtering targets usually take the form of disk-shaped plates. As accelerated particles impinge on a target surface, the atoms constituting the target are partly released by momentum exchange to deposit on a substrate. Typical sputtering targets in use include targets made of Al alloys, refractory metals and alloys (W, Mo, Ti, Ta, Zr, Nb, etc. and their alloys such as W--Ti), and metal silicides (MoSi.sub.x, WSi.sub.x, NiSi.sub.x, etc.). Metal silicides, and refractory metals and their alloys, are particularly useful because of their high melting points, oxidation resistance, and other merits. Among the alloy targets, targets of W--Ti alloys are predominantly used in forming diffusion barriers for various integrated circuit devices of one megabit or higher capacities. It is known that a W--Ti sputtering target may be used to form a W--Ti alloy film as a diffusion barrier for IC devices. For instance, the formation of diffusion barriers by sputtering using a W-10 wt % Ti alloy target has been put into practice. In fact, some types of W--Ti alloy targets are already available on the market.
Metal silicide targets for sputtering, having a silicon/metal molar ratio in excess of 2, are generally made by compacting and sintering a synthetic silicide powder formed by the silicidizing reaction between silicon and metal powders. Their crystal structures comprise binary phases, i.e., silicide and free silicon phases, corresponding to the given silicon molar ratios of the individual silicides. The sizes of the respective phases depend largely upon the particle diameters of the synthetic silicide powder and the silicon powder added for molar ratio adjustment.
A recent problem in the art is the deposition of coarse particulate deposits, commonly known as "particles" on films formed by sputtering. By porticles is meant the deposits which form on a substrate as clusters of particles which have been scattered within a sputtering chamber during the course of thin film formation. The clustered particles often have sizes as large as several microns, and their deposition on the substrate can cause troubles. For example, the particles can cause shorting or even breaking of wirings or interconnections of LSI devices, leading to an increased percentage of rejects. The deposited particles come from varied sources, partly originating from the target itself and partly from the thin film forming system.
Much attention is now focused, above all, upon the particles originating from targets. In the case of metal silicide targets, coarse free silicon phases in the crystal structure of silicide targets have been found primarily responsible for particle generation and evolution. With the view of reducing the rate of particle formation, there is a demand for targets of more homogeneous and finer crystal structures. The demand has been met somewhat by the pulverization of silicide powder and silicon powder prior to their compaction and sintering. This countermeasure is relatively easy to implement. However, when further homogeneity and fineness of target crystal structures is required in the future, problems of process complexity and contamination with oxygen and other impurities will arise. This is because the size reduction of particles involves a vast increase in the specific surface area, in geometrical progression.
Specifically, the attainment of more homogeneous and finer target crystal structures is essential for further reduction of porticle produced from targets in the future. The conventional process that consists of chemically synthesizing a silicide powder from a fine refractory metal powder and a fine silicon powder, and then grinding and classifying the silicide powder to finer powder particles, poses the following problems:
(1) The silicidizing reaction is exothermic, and the finer is the refractory metal powder and the silicon powder starting materials, the faster the reaction progresses, with increasing difficulty of control. PA1 (2) The grinding and classification steps required for fining down the silicide and silicon powders are not practically feasible since the steps are beyond the performance limits of existing ball mills, classifiers, etc. PA1 (3) Fine division of silicide and silicon powders raise concerns over contamination with increased oxygen concentrations, due to the increased specific surface areas of the powders. PA1 (4) Further, in connection with the above oxidation problem, it is to be noted that minimizing the oxidation that inevitably results from compaction and sintering is a key to successful fabrication of high-quality sputtering targets. Tackling the problem from this perspective too seems necessary. PA1 (1) Mechanical alloying provide targets more homogeneous and finer in structure than heretofore known. PA1 (2) A metal silicide powder prepared by mechanical alloying yields, upon compaction and sintering, a high melting temperature silicide target. The target has a homogeneous, fine crystal structure, with free silicon phases 5 .mu.m or finer in size, and with an oxygen concentration of 500 ppm (by weight) or less. This crystal structure has seldom been manufactured by prior art processes. PA1 (3) The particles prepared by mechanical alloying are secondary particles of a metal silicide 10 to 20 .mu.m in particle size. The secondary particles are aggregates of primary silicide particles of the 5 .mu.m or smaller in size. Hot pressing the secondary particles gives a high melting temperature silicide target having a homogeneous, fine crystal structure, with free silicon phases 5 .mu.m or smaller in size and an oxygen concentration of 500 ppm (by weight) or less. Such silicide targets have been difficult to manufacture conventionally. The use of the secondary particles has been found extremely helpful in alleviating the oxidation problem. PA1 (4) The metal silicide and alloy powders prepared by mechanical alloying permit the use of lower hot press temperatures than have been used before. This results in the surprising advantages of reduced oxidation and reduced crystal grain coarsening. Thus, mechanical alloying is beneficial not only to the preparation of metal silicide and alloys but is also beneficial to the prevention of the material's contamination with oxygen and the like, as a result of the use of lower hot press temperatures. PA1 (5) Hot pressing W--Ti alloy powder prepared by mechanical alloying, containing secondary particles, such as particles 10 .mu.m or larger in particle size, yields a target having a homogeneous, fine crystal structure, with Ti-rich phases 50 .mu.m or smaller in size and with an oxygen concentration well within a desirable range that is difficult to obtain by ordinary methods. Hydrogenation and dehydrogenation do not in the least take part in this process. PA1 (6) Mechanical alloying can handle material powders of relatively large particle sizes, and therefore averts the problem of oxygen intrusion which otherwise accompanies the use of fine starting material powders. Moreover, the use of secondary particles is very effective in lessening the oxidation problem. PA1 If the temperature is too low, the powder density will not increase. If the temperature is excessive, the number of particles in Ti-rich phases will decrease, but the oxygen content will increase. PA1 The pressure which may be applied to a press is dictated by the proof stress of the die used. Where a die of high proof stress is available, a correspondingly high press pressure is used. PA1 According to the invention, the sintering temperature may be lower and the pressing time shorter than before, since the W--Ti alloy at the time of hot pressing is already in the form of uniform fine particles. The lower hot pressing temperature and shorter pressing time are great advantages in that they lessen the oxidation that results from hot pressing. The lower hot pressing temperatures and shorter pressing time also prevent coarsening of the crystal grains.
The same oxidation and particle problems exist in alloy targets, especially Ti-containing refractory metal alloy targets. These problems will now be explained using a W--Ti alloy target as an example. A W--Ti alloy target is made by powder metallurgy from a mixture of tungsten and titanium powders. The target is made by either cold pressing followed by sintering, or by hot pressing. Commercial titanium powder, one of the starting materials, has a high oxygen content. This is because titanium is active enough to be readily oxidized, and inevitably has an oxide surface film. Titanium powder in the form of smaller particles with a larger overall surface area has a very large total oxygen content. Targets made by powder metallurgy from a titanium powder with such a high oxygen content are naturally high in oxygen content. Sputtering using an oxygen-rich target is undesirable. The liberation of oxygen can cause problems such as target cracking, oxidation of the resulting film, and loss of uniform film quality. For example, commercially available W--Ti alloy targets contain at least 1240 ppm and in most cases 2500 ppm or more, oxygen. The oxygen content is too large for the formation of high-quality insulation barriers or the like.
Under these circumstances, we conceived the idea of using titanium hydride (TiH.sub.2) powder as a titanium source and dehydrogenating it afterwards. We then proposed, in Patent Application Public Disclosure No.303017/1988, a method of manufacturing a W--Ti alloy target characterized by the steps of mixing tungsten (W) powder with titanium hydride (TiH.sub.2) powder, and hot pressing the powder mixture after or concurrently with its dehydrogenation. The method has made possible the manufacture of W--Ti alloy targets having a lower oxygen content, with reasonably good success.
Nevertheless, sputtering with those targets has caused a new problem; they give off so many particles that they can hardly be used in fabricating devices with wirings less than one micron wide, e.g., 0.5 .mu.m wide. We found some time ago that commercially available W--Ti alloy targets are characterized by a large average particle diameter of their Ti-rich phases, ranging from tens of microns to about one hundred microns. We found that sputtering of such targets allows preferentially spattering of the coarse Ti-rich phases, thus contributing to the large particle production. We then tried using a titanium hydride (TiH.sub.2) powder 60 .mu.m or smaller in particle size as a titanium source, mixing the powder with a tungsten powder, and hot pressing the mixed powder after or simultaneously with dehydrogenation. In this way the particle size of the Ti-rich phases in the W--Ti alloy target was decreased to 50 .mu.m or less, and the particle production was successfully reduced while maintaining a low oxygen content (Patent Application Public Disclosure No.232260/1992). This process, however, requires the preparation of a titanium hydride powder and dehydrogenation of the mixed powder of titanium hydride and tungsten powders. Hydrogenation is a highly hazardous step. A abrupt hydrogen absorption produces a negative pressure inside the oven or reactor used for the powder preparation. Dehydrogenation, which is performed in a vacuum or inert atmosphere at a temperature between 500.degree. and 700.degree. C., demands special care to prevent an increase the oxygen content, and to control the evolving hydrogen.
It is likely that targets in the future will be required to have more homogeneous and finer crystal structures so as to reduce the particle production. Finer division of the raw material powders to satisfy this requirement will be accompanied with a vast increase in the specific surface area, in geometrical progression. The latter, in turn, will present process complication and contamination with impurities such as oxygen. Finer grinding and classification will become practically impossible since. These practices will overburden existing ball mills, classifiers, etc. A manufacturing process that involves mixing currently available titanium source powder with tungsten powder and hot pressing the mixture has its limitations from the viewpoint of further powder size reduction, as in the case of the metal silicide powder discussed before.
There is a demand, therefore, for a way of easily manufacturing metal silicide targets and metal alloy targets that are more homogeneous and finer in crystal structures with less oxygen contents than heretofore possible.